Wireless communication channels suffer from fading, or loss of signal, due to changes in the propagation environment of the wireless signal. Some types of fading, such as Rayleigh fading, can be highly localized in nature. Furthermore, wireless communication systems are often limited in the amount of bandwidth that can be used, due to practical restrictions on the electronics that are used, or due to licensing and regulatory restrictions.
The performance of wireless communications may depend on antenna systems, also known as smart or intelligent antennas. Recently, multiple antenna technologies have emerged to achieve the goals of 4G systems, such as high rate, high reliability, and long range communications. For example, multiple-input and multiple-output, or MIMO, refers to the use of multiple antennas at the transmitter and the receiver end of a wireless link. MIMO technology may offer significant increases in data throughput and/or transmission range without the need for additional bandwidth or transmit power. It can achieve this due to the ability of MIMO to obtain higher spectral efficiency (more bits per second per hertz of bandwidth), link reliability, and/or reduced fading.
MIMO based systems may allow the use of a variety of coding techniques that take advantage of the presence of multiple transmit and receive antennas. For example, wireless communications performed over a MIMO channel can use beamforming, spatial multiplexing and/or diversity coding techniques.
Beamforming involves transmitting the same signal on each of the transmit antennas with appropriate complex (i.e., gain and phase) weighting such that the signal power is increased at the receiver input. The benefits of beamforming may include increased signal gain from constructive interference and reduced multipath fading.
In spatial multiplexing, a high data rate signal is split into multiple lower data rate streams, and each stream is transmitted from a different transmit antenna in the same frequency channel. The receiver separates the received streams and combines the received data streams into a single receive stream, thereby increasing channel capacity.
In diversity coding methods, a single stream is transmitted, but the signal is coded using space-time coding techniques so that the signal emitted from each of the transmit antennas is substantially orthogonal. Diversity coding exploits the independent fading in the multiple antenna links to enhance signal diversity.
To increase performance of a MIMO system, it is desirable for fading on the wireless links between the transmit and receive antennas to be uncorrelated. That is, it is desirable for there to be a low statistical correlation between fading experienced at one antenna and fading experienced at another antenna.
Correlation between antennas can be reduced by causing the antennas to have different polarizations, i.e. sending and receiving signals with orthogonal polarizations. Furthermore, antennas for MIMO systems may utilize spatial separation, or physical separation, to reduce correlation between antennas.
Generally, for frequency division duplex (“FDD”) systems with simultaneous transmit and receive, such as code division multiple access (CDMA), Universal Mobile Telecommunications System (UMTS), 3GPP LTE (3rd Generation Partnership Project Long Term Evolution) and the like, there is a problem with the transmitter reducing the sensitivity of the receiver. The problem is, generally, most significant when the transmitter is working at maximum transmit power. In other words, the problem is most noticeable under poor signal conditions. The phenomenon is usually referred to as “Tx on desense.”
With the introduction of MIMO and diversity, a secondary antenna is added to the system. This antenna is a generally configured as a pure receive antenna (i.e., only used as a receive antenna) and is used to increase the download throughput. However, the modulation of choice and utilization of the secondary antenna will vary depending on the signal conditions. In good signal conditions, the secondary antenna will generally be used for MIMO and, as the signal drops, the usage will be reduced to diversity and, at the cell edge, the higher order data modulation might not be used at all. The gain of the diversity antenna at the cell edge is generally not used, at least for voice communications and, as such, the secondary antenna is not used.
Generally, the determination to use the MIMO communication mode is made by the wireless system based on power readings received from the mobile terminal, such as received signal strength (“RSSI”) and/or received signal carrier power (“RSCP”) measures of received signal power level. When MIMO is not selected for the mobile terminal, the mobile terminal itself generally determines how to use the multiple antennas in the mobile terminal.